Client#signedURL documenation

Hey guys,

I am looking to create a signed URL using Knox Client class, and while I can find links to resources on [StackOverflow](http://stackoverflow.com/questions/6447202/creating-s3-signed-urls-in-node-js) (how I initially came to this feature) and can see it in the [codebase](https://github.com/Automattic/knox/blob/master/lib/client.js#L855), I was surprised that the [documentation](https://github.com/Automattic/knox/blob/master/Makefile#L7) had not made it to the README.md.

Is this intentional? Because I think it would be awesome to better highlight this feature on the project page so that new users like me -- and our robot crawler overlords -- can better direct people to this feature.

Would the project be open to a pull request to add the documentation?

Thanks,
Jacob

Check the signedURL implementation in lib/client.js around line 855 and the documentation build reference in Makefile around line 7. Update README.md to highlight how Knox Client creates signed URLs, then verify the documented feature matches the existing API and generated documentation setup.

Written by the indexing model from the issue text.